1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery such as a lithium secondary battery.
2. Description of the Related Art
In recent years, a nonaqueous electrolyte secondary battery, in which an alloy, metal lithium or a carbon material capable of occluding/releasing lithium ions is used as a negative active material and a lithium transition metal complex oxide expressed by a chemical formula LiMO2 (M is a transition metal) is used as a positive active material, is noted as a battery having a high energy density.
As the above-mentioned lithium transition metal complex oxide, a lithium cobalt complex oxide (LiCoO2) is given as a typical one, and it has already become commercially practical as a positive active material of the nonaqueous electrolyte secondary battery.
In the nonaqueous electrolyte secondary battery in which a lithium transition metal complex oxide such as lithium cobalt oxide is used as a positive active material and a carbon material is used as a negative active material, 4.1 to 4.2 V is generally employed as the end of charge voltage. In this case, the positive active material is utilized only by 50 to 60% with respect to its theoretical capacity. Therefore, if the end of charge voltage is more raised, a capacity (coefficient of use) of a positive electrode can be improved and the capacity and the energy density of the battery can be enhanced.
However, if the end of charge voltage of the battery is raised, the deterioration of a structure of LiCoO2 and the decomposition of an electrolyte solution at the surface of the positive electrode become apt to occur. Therefore, there was a problem that the deterioration of the battery due to charge-discharge cycles becomes more remarkable than the conventional case of employing 4.1 to 4.2 V as the end of charge voltage.
On the other hand, of lithium transition metal complex oxides expressed by a chemical formula LiMO2 (M represents transition metals), compounds containing Mn and Ni as transition metals have been studied and also materials containing all three kinds of transition metals of Mn, Ni and Co have been actively studied as shown in Japanese Patent Publication No. 2561556, Japanese Patent Publication No. 3244314 and Journal of Power Sources 90 (2000) 176-181.
It is reported that among the lithium transition metal complex oxides containing Mn, Ni and Co, a compound in which compositions of Mn and Ni are equal exhibits specifically high thermal stability even in a charging state (high oxidation state) in Electrochemical and Solid-State Letters, 4(12) A200-A203 (2001). Also, it is reported that a complex oxide, in which Mn is substantially equal to Ni, has a voltage of around 4V equal to that of LiCoO2 and exhibits a high capacity and excellent charge-discharge efficiency in Japanese Patent Laid-Open No. 2002-42813.
In such a battery in which a lithium transition metal complex oxide containing Mn, Ni and Co and having a layered structure is used as a positive active material, it can be expected that reliability of the battery is dramatically improved by virtue of high thermal stability during charging even when the end of charge voltage of the battery is raised to deepen the depth of charge capacity of a positive electrode.
However, the present inventors studied on a battery in which the lithium transition metal complex oxide containing Mn, Ni and Co is used as a positive active material and as a result have found that when the end of charge voltage is raised, the deterioration of a structure of the positive active material and the decomposition of an electrolyte solution at the surface of the positive electrode become apt to occur and reduction in a battery capacity due to the charge-discharge cycles becomes more remarkable than the conventional case of employing 4.1 to 4.2 V as the end of charge voltage.
In order to solve the above-mentioned problems, the present applicant has proposed in patent application No. 2004-94475 not laid open that a lithium transition metal complex oxide formed by allowing LiCoO2 to contain at least both of Zr and Mg and a lithium transition metal complex oxide having a layered structure and containing at least both of Mn and Ni as transition metals are mixed and this mixture is used as a positive active material. By using such a mixture as a positive active material, the end of charge voltage can be raised to enhance a charge-discharge capacity without deteriorating a charge-discharge cycle characteristic and thermal stability. The present invention is one which further improves a charge-discharge cycle characteristic in the nonaqueous electrolyte secondary battery in which such the mixture is used as the positive active material.